A device of this type is known from U.S. Pat. No. 4,457,305. The device, hereinafter called “water trap,” comprises an inlet duct and an outlet duct for breathing gas, which are arranged at an angle to one another. A liquid duct, which can be closed by a valve, is located at the deepest point, in order to remove the condensate from the breathing gas duct. The liquid duct opens into a liquid container, which is pushed over a connecting flange. A pin, which points in the direction of the valve and has such a length that a valve body lifts the valve from its valve seat by the pin when the liquid collecting container is connected to the connecting flange, is located at the bottom surface of the liquid collecting container. With the valve opened, the condensate can flow off from the breathing gas duct into the liquid collecting container. If, by contrast, the liquid collecting container is removed from the connecting flange, the valve body lies on the valve seat and the liquid duct is closed. The valve arranged in the liquid duct is also used to close the breathing gas duct against the environment in order to prevent breathing gas from escaping into the environment when the liquid collecting container is removed.
So-called tube-in-tube systems, which have an inner gas duct and an outer gas duct arranged concentrically thereto in order to send inspiration gas to the patient and to take up expired gas, are also used to respirate patients. Since humidified breathing gas is usually used during respiration, condensate, which must be drawn off, may occur in both gas ducts. Even though it would be possible to provide a separate water trap for each gas duct, this would make handling difficult, because two liquid collecting containers must always be checked and optionally removed during the operation. The use of two separate water traps is possible in the coaxial tube system by technically complicated measures only. Thus, the outer gas duct must be sealed during the removal of the condensate from the inner gas duct. The flow characteristic in the outer gas duct may change in this case and lead to an increase in flow resistance there.